10 Important SIP Interview Questions (With Sample Answers)

By Indeed Editorial Team

Published 19 May 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

SIP is a signalling protocol that enables the creation, modification and termination of multimedia sessions through the internet protocol (IP). It is a protocol for the application layer that integrates numerous aspects of the hypertext transfer protocol (HTTP) and the simple mail transfer protocol (SMTP). If you are interested in working in information technology jobs, preparing to answer questions regarding session initiation protocols (SIPs) can help you demonstrate your skills and expertise on this subject to the interviewer. In this article, we list 10 common SIP interview questions and sample answers to help you prepare for your interview.

10 common SIP interview questions with sample answers

Here are some common SIP interview questions the interviewer may ask in the interview:

1. What SIP is and how is it used?

This interviewer may ask such questions early in the interview to ascertain your broad understanding of SIP and why it is necessary. Briefly define what a SIP is and expand on your response by providing a concrete example of how you can apply SIP to a real-world application.

Sample answer: "SIP is one of the most common protocols used in voice-over-internet (VoIP) technology. It is an application layer and text-encoded protocol which works with multimedia sessions to request and deliver instant messages. SIPs specify the format of messages being exchanged and establish communication sequences between session participants. A user may, for instance, send a message to a group chat. The session initiation protocol communicates the text message application's actions. It enables the user to start the application's message sending process."

2. What are the major functions of a SIP protocol?

The interviewer may ask such a question to find how well you understand the SIP's practical use case. While answering such a question, it can be beneficial to be more specific about how SIPs work to demonstrate a greater knowledge and understanding. It may assist you in establishing your industry knowledge and ability to apply concepts to practical scenarios.

Sample answer: "Some of the major functions of a SIP protocol includes locating clients and resolving SIP to the IP address, discussing capabilities and features among session participants, modifying session parameters and helping with setup and teardown of calls for all clients during a session."

3. How do transactions, dialogues and sessions differ from each other?

The interviewer may ask such questions to ascertain your familiarity with various industry-specific terminology. Explaining these key terms demonstrates that you are familiar with the industry's major phrases and can describe them clearly and concisely. To answer such a question, define each of these concepts. Then contrast them to demonstrate how they differ in the context of session initiation processes.

Sample answer: "The terms transactions, dialogs and sessions, all refer to distinct aspects of media exchanges while using a SIP. A transaction, for example, is the fundamental unit of message exchange, describing the request-response cycle. Applications generate dialogs in response to non-failure responses to requests. They are user-to-user interactions when two users commence a session within an application. A session is a media interaction between two or more endpoints, like when many users join a session. Sessions and dialogues are self-contained."

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4. What is the importance of the Call-ID header field?

As SIPs use a variety of distinct header fields, the hiring manager may ask you to describe the relevance of various fields to demonstrate your understanding of what each of these fields performs. Your answer to such a question indicates your comprehension of the header and its application to SIPs. To answer such a question, briefly explain the importance of the call-id header field.

Sample answer: "A common type of header information in a SIP INVITE is the Call-ID header field. SIP INVITE is the initial signal sent by SIP when a user initiates a session, such as by dialling another user. A Call-ID is a unique identifier used to identify and monitor distinct sessions. When a user initiates a session, the Call-ID generates a locally unique string that differentiates the session from others. This header enables each session to develop a unique identity that distinguishes it from other sessions, allowing both parties to differentiate the call from others."

5. What do you mean by the proxy server and why is it used?

This may be one of the most common questions an interviewer ask during any networking related interview. To answer this question, define the proxy server and its use in the context of SIPs.

Sample answer: "A SIP proxy server lies at the heart of the SIP message exchange, receiving and forwarding messages, but not initiating or terminating sessions. A proxy server typically has access to a database or a location service to assist it in processing the request from figuring out the subsequent hop. Databases may wish to include SIP registrations, presence data or other information based on the location of a client."

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6. How does call transfer differ from call redirect?

The hiring manager may ask such questions to assess your attention to detail on two similar-sounding concepts. To answer this question, briefly explain call transfer and call redirect.

Sample answer: "In a call transfer a SIP user agent (UA) establishes a dialogue with the caller and then commence setting up a new dialogue between the caller and another UA wheres in a call redirect case, a UA does not answer the call but only informs the caller to resend the INVITE to another SIP URI."

7. What is the role of the Session-Expire header field?

Interviewers ask such questions to ascertain your level of familiarity with session intervals in SIPs. A good way to answer such questions is to briefly explain what session-expires headers are and how they work.

Sample answer: "The Session-Expires header field indicates how long a SIP session will last. It is included in INVITE and UPDATE requests and any 2xx response to an UPDATE or INVITE. Both user agents are informed that a session timer is active when it is supposed to expire and who is renewing the session via the Session-Expires header field in the response.

Before the session expires, the active refresher creates a session refresh request, which is an UPDATE or re-INVITE request. In case the refresher does not receive a response to the session refresh request, it terminates the session with a BYE."

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8. What advantage SIP URI offers over IP address?

The hiring manager may ask such questions to ascertain your ability to weigh the relative merits of several types of protocols. This question allows you to demonstrate your understanding of why SIP uniform resource identifiers (URI) are beneficial to users and how IP addresses compare. This question allows the hiring manager to ascertain your SIPs and IP addresses knowledge.

Sample answer: "A request routed solely through IP addresses will reach a single endpoint device. Since communication is often user-to-user rather than device-to-device, a more useful addressing technique allows one user to call another, ensuring that the request reaches the intended user regardless of which device they are currently using if they own multiple devices. A SIP URI is a name that resolves to an IP address during the call using a SIP proxy server and domain name system (DNS) lookups."

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9. What is the role of the TURN protocol?

Network address translation (NAT) topologies are a key component in communication over the internet. This is one of the most basic questions an interviewer ask during a SIP interview. To answer, mention the TURN protocol's advantages in practical uses.

Sample answer: "Traversal using relay NAT (TURN) protocol enables a client to obtain a transport IP address and port using which it can obtain packets sent from a single IP address within the public internet. For some NAT topologies that contain a listener behind a symmetric NAT, utilising a relay situated within the public internet is the most convenient method for allowing voice communication to occur."

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10. What is the session description protocol (SDP)?

Answering this question allows you to show your understanding of one type of SDP framework. When you answer this question, explain what the SDP is and why it is important in SIPs, along with some SDP examples.

Sample answer: "The session description protocol is a standardised framework for defining multimedia communication sessions for announcement and invitation reasons. It is commonly used with streaming media applications such as voice over IP and video conferencing. An SDP session description includes the media information, such as media type–audio or video, transport protocol–RTP/UDP/IP or H.320, media format–H.261, MPEG, G.771 or G.729."

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